The cell biology of Leishmania is investigated as a model of both intra- and extracellular parasitism. Emphasis is placed on characterizing the biochemical and physiological functions of its surface membrane (SM) and secreted components toward defining their roles in parasite survival and development. Leishmania promastigotes possess a number of SM transport systems for the active accumulation of certain essential amino acids and nucleosides. One of these, a putative SM adenosine transporter was identified in situ in L. donovani promastigotes using several radiolabeled, affinity-binding, cross- linking reagents. This externally oriented SM protein has an apparent molecular weight of 30 kDa is currently being characterized. Both molecular and immunochemical approaches are being used to identify genes coding for the 2 leishmanial L-proline transport systems. Leishmania and Crithidia were both shown to use S-adenosyl-methionine as a protein methylating agent. In both of these trypanosomatids, methylation appears to be a mechanism for modifying/regulating the activity of certain SM proteins in situ. The SM 3'-nucleotidase of L. donovani promastigotes was purified to homogeneity. The substrate specificity, metal cation requirements and inhibitors of this 38 kDa glycoprotein were characterized. Three separated glycosylation events were identified during the biosynthesis of the leishmanial secretory acid phosphatase (SAcP). The last of these includes the Golgi-bases addition of [PO4-6Galbeta1,4Man]n, the repetitive, constitutive epitope of the SM lipophosphoglycan (LPG). Terminal stages in LPG biosynthesis were also shown to occur in the Golgi compartment. Genetic analyses of subtractive libraries is being used to identify genes involved in LPG expression. cDNA clones are being sequenced to identify the full length copy of the SAcP gene. In vitro cultivated amastigotes are being used as a model of parasite gene regulated differentiation and development.